Currently, production of a super-high-performance low-cost display requires both of (i) TFT (Thin Film Transistor) performance equivalent to that attained with the use of monocrystalline silicon and (ii) a reduction in manufacturing cost. According to conventional TFT processes such as a process using amorphous silicon (hereinafter referred to as “a-Si”) and a process using polycrystalline silicon (hereinafter referred to as “poly-Si”), a TFT having desired performance cannot be obtained. In addition, since such conventional TFT processes require huge devices such as a vacuum device, a laser crystallization device, and an exposure device, it is difficult to reduce a manufacturing cost.
For example, (a) of FIG. 13 illustrates a case where circuit elements such as pixels are formed on a large-area glass substrate by a conventional TFT process (an a-Si process or a poly-Si process) for a large-sized liquid crystal display device. This method necessitates forming an a-Si film or the like onto an entire surface of the glass substrate and crystallizing the entire surface of the substrate with the use of a laser. Accordingly, as the size of mother glass increases (size of tenth generation mother glass: 3.1 m×2.9 m), a huge device and enormous investment are required. It is therefore difficult to reduce a manufacturing cost. Moreover, according to this method, only TFTs which largely vary from each other in performance and which require a large amount of power can be obtained.
In view of this, methods were developed in which a super-high-performance low-cost display is produced by (i) forming elements on a small-area substrate and then (ii) causing the elements thus formed to be mounted on a large-area glass substrate so that the elements are distributed in an array. For example, (b) of FIG. 13 illustrates an example of a method in which (i) silicon devices or circuit elements are formed on a silicon substrate (hereinafter referred to as “Si substrate”) by an existing IC (Integrated Circuit) process, (ii) the silicon substrate on which the silicon devices or the circuit elements are formed is divided into chips, and (iii) the chips are transferred (or bonded) onto a large-area glass substrate. The transfer may be performed by a Smart Cut process (hydrogen ion separation process) or may be performed by bonding using die bonding.
Transfer of elements using the Smart Cut process is disclosed, for example, in Patent Literatures 1 and 2 invented by the inventors of the present invention. This method is effective in a case where the substrate is divided into approximately tens to hundreds of chips (each having a size of several millimeters) as in a case of a driver of a panel. However, dividing the substrate into millions of elements (each having a size of tens of micrometers) and bonding the elements as in the case of pixel TFTs is impractical not only in terms of throughput but also in terms of handling.
(c) of FIG. 13 illustrates a case where a Si substrate or silicon devices (elements) that are formed on a Si substrate (Si wafer) by the Smart Cut process are bonded to a large-area glass substrate without dividing the Si substrate into chips. According to this method, it is unnecessary to divide the Si substrate into a plurality of chips unlike the case of (b) of FIG. 13. However, according to this method, the silicon devices are separated from the Si substrate by thermal treatment after the silicon devices are bonded to the glass substrate. Accordingly, all of the elements on the Si substrate are transferred onto the glass substrate. Consequently, the silicon devices cannot be transferred while securing a wide spacing, such as pixel pitch, between the silicon devices. In order to transfer elements while securing a wide spacing between the elements, it is necessary to form the elements on a silicon substrate so that the elements are disposed away from each other as illustrated in (d) of FIG. 13. However, this causes a large decline in use efficiency of the silicon substrate.
Further, according to conventional techniques, it is very difficult to transfer high-performance silicon devices or circuit elements formed on a silicon substrate by a single transfer process so that the silicon devices or the circuit elements are distributed. For example, according to the Smart Cut process, which can be used to transfer monocrystalline silicon devices, the whole substrate is heated. Accordingly, it is difficult to selectively transfer only desired monocrystalline silicon devices.
Meanwhile, as a method for selectively distributing elements in order to reduce a manufacturing cost, methods such as the methods disclosed in Patent Literatures 3 and 4 are known. According to the methods disclosed in Patent Literatures 3 and 4, minute elements formed on a small substrate are transferred onto a large-area substrate so that the elements are distributed on the large-area substrate. The following describes an example of how the elements are transferred with reference to FIG. 14.
First, a release layer 12 is formed on a first substrate (base substrate) 11, and a plurality of elements 13 are formed on the release layer 12 (see (a) of FIG. 14). Then, for example, after the elements 13 are bonded to an intermediate substrate 15 coated with a UV release adhesive 14, the first substrate 11 and the intermediate substrate 15 are detached from each other along the release layer so as to transfer all of the elements (first transfer process) (see (b) and (c) of FIG. 14). The detachment is carried out by a method such as a liftoff method using wet etching and an etch stopper layer or a laser abbration of irradiating the release layer with a laser from a rear surface of the first substrate 11. In the case of irradiating the release layer with a laser from the rear surface of the first substrate 11, the first substrate 11 need to be a transparent substrate. Next, the transparent intermediate substrate 15 on which the elements 13 are retained (see (c) of FIG. 14) is bonded to a final substrate (transfer destination substrate) 17 coated with an adhesive 16 (see (d) of FIG. 14). Then, the UV release adhesive 14 is selectively irradiated (irradiated at fixed spacing which corresponds to a pixel pitch) with UV light from a rear surface of the intermediate substrate 15 so as to weaken adhesion of the UV release adhesive 14 (see (e) of FIG. 14). Thereafter, the intermediate substrate 15 and the final substrate 17 are detached from each other, so as to transfer only desired elements 13 onto the final substrate 17 (second transfer process) (see (f) of FIG. 14). Also in a case where the UV release adhesive 14 is irradiated with UV light from the rear surface of the intermediate substrate 15 so as to perform the second transfer process, the intermediate substrate 15 need to be a transparent substrate.